Field of the Invention
The invention relates to a heatset printing ink for the printing of documents by intaglio printing, i.e., engraved steel die printing. The ink has been found to be especially useful for the printing of security documents such as stamps, checks, stock certificates, bank notes, tickets, etc.
The term xe2x80x9cintaglio printingxe2x80x9d refers to a printing process wherein a printing cylinder or printing plate carries the engraved pattern and the engraved recess is filled with printing ink to be transferred to the printing substrate in order to create the document. In this type of printing, typically a rotating engraved cylinder (usually manufactured from steel or nickel and plated with chromium) is supplied with ink by one or more template inking cylinders by which a pattern of inks of different color is transferred to the printing cylinder. Any excess ink on the surface of the cylinder is then wiped off by a rotating wiper cylinder covered by a plastisol, using a dilute aqueous solution of sodium hydroxide and sulfonated castor oil as an emulsifying medium for the wiped-off excess ink. Thereafter, the printing pattern is transferred, under a pressure of up to 105 kg/cm, to the substrate.
The most widespread process for printing security documents, especially currency, is sheetfed non-heatset sheetfed intaglio. Sheetfed non-heatset intaglio inks are based on oxidizable resins and alkyds and are very slow drying. Typically, one side of the currency is printed first and after 24-48 hours, the other side is printed. The typical printing speed of sheetfed intaglio is about 45-75 m/min. U.S. Pat. No. 4,966,628 discloses sheetfed intaglio inks suitable for printing of security documents.
Recently, patents disclosing heatset intaglio printing inks suitable for printing of security documents have issued to the same assignee of the present invention, e.g. see U.S. Pat. Nos. 5,100,934 issued Mar. 31, 1992 (hereinafter the xe2x80x9c""934 patentxe2x80x9d) and 5,367,005 issued Nov. 22, 1994. Both patents describe printing ink formulations, which have proven to provide excellent performance in respect to the heatset intaglio printing of currency. However, the inks disclosed in both patents exhibit either relatively high percentages of volatile organic compounds (xe2x80x9cVOCxe2x80x9d) or are able to tolerate only small amounts of water.
For example, the xe2x80x9ccuring agentxe2x80x9d of the ""934 patent is an amine selected to promote the crosslinking of the resin, in effect the formation of a polymeric network of the resin for certain resistance properties in its intended application, with diamines being preferred. The curing agent disclosed by the ""934 patent when used as a neutralizing agent forms a resulting crosslinked resin or polymeric network which offers little water tolerance.
Thus, until the present invention, efforts to reduce the VOC content of the heatset intaglio inks, or increase the water tolerance of such inks, have failed. Reducing the content of volatile solvents or substituting water for part of the volatile solvents have resulted in incompatibility problems because of the nature of the resins present in such inks.
It has now been found that it is possible to replace part of the VOCs in heatset intaglio printing inks with water, thereby dramatically reducing the VOC content of such inks, while at the same time avoiding incompatibility problems, which would otherwise occur because of the presence of water in such inks. The solution to this problem is that a monoalkanolamine is employed in the inks of the present invention, instead of the diamine curing agents employed in the inks disclosed in the ""934 patent. As will be apparent from the examples set forth below, the tolerance of the heatset intaglio printing inks for water is significantly and surprisingly improved, up to 50-100 fold over the prior art intaglio printing ink compositions (e.g. the ""934 patent) as a result of the presence of the alkanolamine.